1. Field of the Invention
The present invention relates to a silicon wafer for hydrogen heat treatment and the method for manufacturing the same.
2. Description of the Related Art
As the substrates for forming semiconductor elements, high-purity silicon single crystals are mainly utilized. The silicon single crystals are usually manufactured by the CZ method. In the CZ method, a lump shaped polysilicon is fed into a crucible disposed in a single crystal-manufacturing apparatus, and then the polysilicon is heated and melted by heaters surrounding the crucible to form a melt. Thereafter, a seed crystal mounted on a seed chuck is dipped into the melt. The seed chuck and the crucible are then rotated in the same or reverse direction while pulling the seed chuck to grow a silicon single crystal (hereinafter referred as CZ-Si single crystal) having a predetermined diameter and length.
In recent years, with the scaling-down and higher integration of semiconductor devices, the gate oxide film insulation characteristic has particularly received attention. As a means for reducing the defects entrapped in the oxide film in the gate oxide film-forming process, according to JP-A3-80338(JP-A: Unexamined Japanese Patent Publication), it has been proposed that immediately before the formation of a thermal oxide film on the surface of a silicon wafer, the silicon wafer is subjected to a heat treatment at a temperature above 1100.degree. C. in an non-oxidizing atmosphere containing a hydrogen gas. By means of the hydrogen heat treatment, the natural oxide film on the surface of the silicon wafer is removed and hydrogen is bonded to an unsaturated bond of the surface of the silicon wafer.
Moreover, subjecting the silicon wafer to the hydrogen heat treatment eliminates the octahedral void like defects near the surface of the wafer, which are detected as the grown-in defects formed in the crystals during the growing of the CZ-Si single crystals, such as LSTD (Laser scattering Tomography Defects), FPD (Flow Pattern Defects) and COP (Crystal Originated Particles). And thus the thermal oxide film formed thereafter demonstrates good oxide film insulation characteristics.
Table 1 summarizes the results of the measured oxide film insulation of 14 mirror-polished wafers. The 14 mirror-polished wafers are respectively sliced from 14 CZ-Si single crystal ingots which are respectively grown by 14 different growing conditions using a boron doped, p-type, &lt;100&gt;, 150 mm diameter CZ-Si single crystal. The above 14 conditions involve different furnace hot zones and different pulling speed during the pulling of the single crystals. In the measurement of the oxide film insulation, a MOS capacitor is formed on the wafer, the electric field is then raised to 0.5 MV/cm step by step to cause the gate electrode to be in an electric charge accumulation state with respect to the substrate. Thereafter, the electric field when the current flowing in the MOS capacitor reaches 10 .mu.A, is considered to be the insulation breakdown electric field, and if the value is larger than 8 MV/cm, the element is regarded as non-defective article. Moreover, in Table 1, the oxide film insulation yields are set forth with respect to: the state after the mirror-polishing process of the silicon wafer, i.e., in an as-grown state, the state after subjecting to heat treatment in a 100% hydrogen atmosphere at 1200.degree. C. for 1 hour, and the state wherein the surface of the hydrogen heat treated silicon wafer is polished to a depth of 3 .mu.m.
TABLE 1 ______________________________________ Oxide Film Insulation Yield (%) After hydrogen After polishing No. As-grown heat treatment 3 .mu.m ______________________________________ 1 39.2 100.0 43.1 2 34.5 100.0 45.3 3 43.0 99.0 43.7 4 30.4 100.0 36.2 5 52.1 98.2 57.1 6 55.3 96.0 63.6 7 34.2 99.8 44.8 8 27.6 100.0 99.2 9 36.4 97.2 97.8 10 24.8 100.0 99.5 11 19.0 100.0 100.0 12 20.4 99.0 95.2 13 23.7 98.5 96.0 14 22.5 100.0 98.8 ______________________________________
As shown in Table 1, in the as-grown state, the oxide film insulation yield is at most 20-55 percent, however, when subjected to the hydrogen heat treatment as proposed in JP-A 3-80338, the oxide film insulation yield is increased to close to 100 percent, regardless of the growing conditions of the CZ-Si single crystals.
However, in order to ascertain to how deep the effect of the hydrogen heat treatment is from the surface of the wafer, the hydrogen heat-treated surface is polished to remove 3 .mu.m. As a result, it can be classified into the group in which the oxide film insulation characteristic is significantly reduced and returned to the as-grown state, as shown in Nos. 1-7 and the group in which the effect of the hydrogen heat treatment is maintained, as shown in Nos. 8-14. Namely, the effect of the hydrogen heat treatment is limited to the proximity of the surface of the wafer, and can extend to a depth of more than 3 .mu.m from the wafer surface.